Wearable active orthotic devices can be used to amplify the residual intention to extend or flex a joint of patients recovering from neuromuscular deficiencies arising from conditions including stroke, traumatic brain injury and multiple sclerosis, or patients recovering from complex orthopedic injuries. Such orthotic devices can be attached across various joints to which movement assistance is provided, such as across a knee, elbow or ankle. The active orthotic device typically has a first portion that attaches to the patient on one side of the joint, a second portion that attaches on the other side of the joint, and an actuator that movably couples the first and second orthotic portions to provide force(s) to assist with movement of the joint. For example, in a knee augmentation device, the first portion is attached around the thigh and the second portion is attached around the calf. The intention to extend the joint may be sensed by a foot pressure sensor.
Considerable force may be required when assisting a joint such as the knee or elbow. The requirements for the actuator are difficult to provide in a compact, lightweight, battery-operated, wearable device.
Many assistive devices use actuators in which a motor is coupled to a lead screw, which may be an Acme screw or ball screw. The lead screw provides both a rotary to linear motion transformation as well as a gear reduction. It may take 10's of rotations of the ball screw to flex the joint less than 180 degrees, thereby providing an effective gear ration that may typically fall in the range of 20:1 to 100:1. The total ratio to the motor may be further increased by using a gear reduction or pulley coupling with different diameter pulleys. The use of a ball screw can meet the basic requirements, but has several disadvantages. The stroke of a ball screw is determined by the length of the screw and the size of the actuator cannot be reduced beyond the length necessary to supply the stroke required to link with the orthotic device. If the linkage is changed to provide the same range of motion with a shorter stroke, the force of the linear actuator must be increased and that may exceed the strength of the screw or available torque of the driving motor.
Other actuators have a difficult time meeting the output torque requirements while keeping size and weight low. If the actuator uses direct gearing, such as planetary gears, spur gears, or harmonic drive, the final gear must supply the entire torque and requires a large, heavy gear. Direct gearing also does not have a mode in which the drive mechanism is completely decoupled from the output linkage. Such decoupling is highly desirable for rehabilitation robotics in which the patient should be allowed free swing of the leg or arm in between the times when the powered assistance lifts, supports and/or assists the patient. Further, direct drive has a single gear ratio. In powered assistance of the knee, smaller motors can be used if the drive mechanism has different drive ratios to accommodate the need for high torque (for sit-to-stand or stair ascent) when the knee is bent near 90 degrees, and to provide higher speed (with less torque) for fast walking when the knee is nearly straight.
What would be desirable, but is not provided by the prior art, is an actuator that obtains high force without ball screws or large output gears, allows free movement of a patient when no movement assistance is desired, and varies the drive ratio during the stroke.